bzoj 1069: [SCOI2007]最大土地面积

 1 #include<cstdio>
 2 #include<iostream>
 3 #include<cstring>
 4 #include<algorithm>
 5 #include<cstdlib>
 6 #include<queue>
 7 #define M 100005
 8 #define ll long long
 9 using namespace std;
10 ll read()
11 {
12     char ch=getchar();
13     ll x=0,f=1;
14     for(;ch<‘0‘||ch>‘9‘;ch=getchar())
15       if(ch==‘-‘)
16         f=-1;
17     for(;ch>=‘0‘&&ch<=‘9‘;ch=getchar())
18       x=x*10+ch-‘0‘;
19     return x*f;
20 }
21 struct data
22 {
23     double x,y;
24 }a[M],s[M];
25 data operator -(data a1,data a2)
26 {
27     data a3;
28     a3.x=a1.x-a2.x;
29     a3.y=a1.y-a2.y;
30     return a3;
31 }
32 double operator *(data a1,data a2)
33 {
34     return a1.x*a2.y-a1.y*a2.x;
35 }
36 double dis(data a1,data a2)
37 {
38     return (a1.x-a2.x)*(a1.x-a2.x)+(a1.y-a2.y)*(a1.y-a2.y);
39 }
40 bool cmp(data a1,data a2)
41 {
42     int t=(a1-a[1])*(a2-a[1]);
43     if(t==0)
44       return dis(a1,a[1])<dis(a2,a[1]);
45     return t<0;
46 }
47 int n,tot;
48 double ans;
49 int main()
50 {
51     n=read();
52     for(int i=1;i<=n;i++)
53       scanf("%lf%lf",&a[i].x,&a[i].y);
54     int q=1;
55     for(int i=2;i<=n;i++)
56       if(a[i].y<a[q].y||(a[i].y==a[q].y&&a[i].x<a[q].x))
57         q=i;
58     swap(a[1],a[q]);
59     sort(a+2,a+n+1,cmp);
60     s[1]=a[1];
61     s[2]=a[2];
62     tot=2;
63     for(int i=3;i<=n;i++)
64       {
65           for(;tot>1&&(a[i]-s[tot-1])*(s[tot]-s[tot-1])<=0;tot--);
66           s[++tot]=a[i];
67       }
68     s[tot+1]=s[1];
69      for(int x=1;x<=tot;x++)
70       {
71           int a=x%tot+1,b=(x+2)%tot+1;
72           for(int y=x+2;y<=tot;y++)
73             {
74                 for(;a%tot+1!=y&&(s[y]-s[x])*(s[a]-s[x])<(s[y]-s[x])*(s[a+1]-s[x]);a=a%tot+1);
75                 for(;b%tot+1!=x&&(s[b]-s[x])*(s[y]-s[x])<(s[b+1]-s[x])*(s[y]-s[x]);b=b%tot+1);
76             ans=max(ans,(s[b]-s[x])*(s[y]-s[x])+(s[y]-s[x])*(s[a]-s[x]));
77           }
78       }
79     printf("%.3lf",ans/2);
80     return 0;
81 }